Process of producing high purity arsenic trichloride from a slurry of as2o3



July 13, 1965 3,194,631

PRocEss 0F PRODUCING HIGH PURITY ARSENIC TRIcHLoRIDE Lnwaz... A TA1 E Fl LOM Ev. Bmw 0mm CSM BAM .Ml 6mm.. F

INVENTORS. George 5. 606e/ KN .b ANGQWQ.

United States Patent O PROCESS F PRODUCING HRG-H PURITY ARSENIC The invention herein described is an improved method of making arsenic trichloride. A

Arsenic trichloride (AsCl3) has extensive industrial and laboratory uses. Among such uses is that of an intermediate, e.g., ias a reactant with diphenyl oxide to produce -ch1orophenoxarsine, widely used as an insecticide and fungicide.

As in a number of other manufacturing operations, the economics thereof are much dependent on the cost and availability of the necessary beginning materials. Heretofore, neither the eiciency of known methods of preparing AsCl3 nor the quality of the end product have been fully satisfactory.

A need, therefore, has existed for an improved method of making AsC13. The invention has fulfilled this need.

The steps for carrying out the method of the invention are described hereinbelow in reference to the annexed drawing showing a owsheet, and by examples which are illustrative thereof, and are succinctly defined in the appended claims.

The invention is an improved method of making high purity AsCl3 which consists of the steps of (1) slurrying As203 in water or aqueous hydrochloric acid, (2) passing hydrogen chloride through the resulting slurry to produce AsCl3 in suspension, (3) allowing the reaction mixture to stand and stratify into substantially two layers: an

.upper aqueous phase saturated with AsC13 and containing some HC1 and a lower liquid phase consisting of substantially al1 AsC13, (4) separating the two phases, e.g., by drawing off the lower phase or decanting olf the upper phase, and (5) removing substantially all of the AsCl3 from the aqueous phase, by solvent extraction, and optionally recovering AsCl3 from the solvent portion of the extraction, as by evaporation, if desirable, or retaining the solvent extracted solution for use as it is.

In a preferred embodiment of the invention, the process is carried out in a semi-continuous manner wherein the aqueous phase produced in Step 3 above is divided into two portions, one portion being admixed with additional hydrochloric acid `or HC1 gas, as needed, to slurry additional As203 and only the other portion of the aqueous phase being subjected to solvent extraction to remove substantially all the AsC13 therefrom.

As-2O3 may be prepared from a number of sources and by a number of methods. One often employed is that of roasting (i.e., heating in air) arsenopyrite, which is chiefly FeAsS and recovering the thus oxidized arsenic.

The temperature employed for the reaction between the A5203 and HC1 in the aqueous medium in the .practice of the invention is between room temperature and about 100 C. Since the reaction is exothermic, it is usually started at room temperature or slightly above rand thereafter the self-generated heat is controlled at between about C. and about 75 C.

The upper aqueous layer (which may contain up to about 12% of the AsC13 theoretically possible based on the As203) usually consists of about 10 to 20% by weight 'AsC13. The lower AsCl3 layer is free of water, tests sensitive to as little as 1 p.p.m. have not indicated the presence of any water. If substantially pure A5203 is employed and fully reacted with HC1, there will be no other materials than AsC13, water, and some unreacted HC1 present in the products made.

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The entire quantity of the aqueous solution containing AsCls is not usually employed in the preparation of additional batches or in a continuous process because there would thereby be provided an unwanted accumulation of water. The portion of the aqueous acidic solution of AsCl3 used in succeeding reactions of the process is usually about one-half that produced in the preceding reaction, although the amount may be greater or less so long as the desired acid concentration is maintained.

The AsCl3 in that portion of the aqueous acidic solution which is not reused in the process is recovered as aforesaid, from the aqueous acid medium by extracting it with a suitable organic solvent. Any aliphatic or aromatic solvent may be used so long as it is substantially water-immiscible and the AsCl3 is soluble therein. The ratio of solvent to aqueous ASC13 usually employed is about 1:1 by volume but may be varied according to circumstances. An ether is commonly employed as the solvent and particularly diphenyl oxide (or diphenyl ether) which offer certain advantages over other ethers. After extraction, the organic solvent can then be removed from the AsC13 if desired. However, when an ether solution thereof is obtained, it need not be separated from lthe AsClB because it may be useable Ias it is for important uses. For example, when diphenyl oxide is employed, the resulting AsCl3 solution is useful in the manufacture of l-chlorophenoxarsine. When such solution is Vsubjected to proper reaction conditions, 10chlorophenoxarsine is formed in accordance with the following equation:

It is preferred to perform the ether extraction step under HC1 gas pressure wherein the aqueous AsCl3 is saturated with HC1 since the solubility of AsC13 in ether is directly proportional to the HC1 concentration.

A small amount of water, about 0.1% by weight when the extraction is carried out in a HC1 atmosphere, may remain in the ether extract. This can conveniently be removed, if desired, by passing the ether extraction through a dessicant such as CaCl2 or through Molecular Sieves, procurable from The Linde Company, New York, New York, or Micro Traps, procurable from W. R. Grace and Company, New York, New York.

The annexed drawing schematically shows one mode of practicing the process of the invention in a continuous process employing ether as the extractant.

The following procedure is illustrative of the practice of the invention in a semi-continuous process: apparatus is set up in the manner suggested by the flow sheet of the drawing wherein ether is employed as the extractant. It comprises: a suitable closed reaction vessel designated 2 provided with stirrer 4, condenser 5, cooling coil 6, inlet tube 7 from HC1 gas source 8 which extends to a point near the bottom of the vessel, an inlet tube from a solid particulated AszOs source 9, an inlet tube 10 from a twater source 1l, an outlet tube `12, at the bottom leading to storage and outlet tube 14 intermediate the bottom and top of vessel 2 leading to extraction vessel le (described more fully hereinbelow). Extraction vessel lo is provided with two other inlets: tube 20 leading from ether source 2?. and tube 24 leading from 'HC1 gas source 3, `and two outlets: tube 26 located intermediate the top and bottoni of extraction vessel 16 leading to dryer 2S for removing water from the ether solution of ASC13, and a second outlet 30 which bifurcates into two branches: one branch 32 which leads to waste disposal and the second branch 34 having pump 35 therein, which constitutes a recycle system for returning liquid je from the` lower part of extraction vessel V16 to the `upper part thereof. Tube 36 leads from dryer 28 to storage. All tubes `are provided with appropriate valves, designated V, for dow control.

The invention may be carried out by a semi-continuous procedure or'it may be carried out in a batch process. When following a semi-continuous procedure, apparatus of the nature of that shown in the drawing is assembled and necessary sources of As203, HC1, waterand ether are provided. The stirrer is started and water and As203 are admitted into the reaction vessel. HC1 gas is then admitted until a highly concentrated hydrochloric acid (eg, 20% to point of saturation) is produced in vessel 2. Since water will be produced by the ensuing reaction between A-s203 and HClthe inflow of water is then discontinued. Additional As203 is admitted and slurried in the so produced acid solution while continuing the admission of HC1 gas. The As203 reacts with the HC1 to form two liquid phases: a lower AsC13 phase, at least about 99% pure AsC13, and an upper aqueous phase consisting by weight of about 15% to 20% dissolved AsCl3. As the reaction proceeds, a vaporous mixture of HC1, AsC13, and water is present in the reaction Vessel above the slurry. It is recommended that a condenser beV provided having a small vent thereinfor condensing and returning a substantial portion of this yvaporous mixture to the liquid reaction mixture in the reaction vessel. Since HC1 gas, is more soluble in water at lower temperatures and the reaction is exothermic, it may be desirable to provide a cooling means for the reaction mixture. The admission of HC1 and As203 and stirring are periodically temporarily stopped. Suicient of the lower AsCl3 layer Vis drawn oit, as through tube 12, to storage 4to maintain the desired level thereof in the reaction Vesselv and sutlcient of the upper aqueous layer is drawn off into an extraction vessel such as vessel 16Vwhere it is treated with a suitable water-immiscible'organic solvent for AsCl3 for a time suicient to extract substantially all the AsC13 from the water. Improved extraction rate may be provided by means of a stirrer but it is preferred to recycle some of the lower aqueous phase into the upper organic phase. The aqueous portion from the extraction vessel is periodically drawn olip to waste. The upper `organic phase containing extracted AsCl3 is drawn oif and passed through a dryer to remove traces of remaining water, and is placed directlyin storage for use.

The following example illustrates the practice of the` invention by'a batch process employingstandard laboratory equipment: 132 grams (7.32 moles) of water at roomv temperature were placed in a 3-neck, 100G-liter fiask, equipped with a stirrer, reflux condenser, and inlet tube leading to a point near the bottom of the water. Suficient HC1 gas to make a 36% by weight hydrochloric acid was admitted by way of the inlet tube. Stirring was started, 482 grams (2.44 moles) of As203 were then i based on the weight of As203 employed, .and a lower phase consisting of about 99% AsCls, no detectable water, and comprising about 90% of the AsCls theoretically possible based on the weight of As203 employed. Agitation was then stopped andthe apparatus disassembled suciently-to permit the upper aqueous phase to be clecanted. About Vone-half of this was placed in a vessel provided with a HC1 gas inlet line and containing a volume of diphenyl oxiderequal to about the volume of` the aqueous phase. The resulting mixture was shaken two (3) allowing fthe-reaction mixtureY to coo-l times while passing HC1 gases therethrough; An upper ether phase containing AsCl3 was thereby formed which was decanted off and any entrapped water subsequently removed by. drying. Y

As an alternative mode of recovering AsC13 from the aqueous phase-'instead ".rf'shakingV the aqueous phase from the reaction vessel in ether, is to place it in a suitable vessel provided with inlet lines for the solvent, eLg., ether, and HC1 gas and provide controlled agitation or a recycle pump, periodicallyallow theV contents to stratify, and draw oit theupper stratumv comprising AsCl3 in ether, and usually some (eg. 1%) entrapped water which is removed by passing through a dryer. The ether containing the AsCl3 may then be stored for use, as in making V10-chl'orophenoxarsine or, if the `ether solutionY is not .a desired end product, the ether extraction phase can be distilled to separate the AsCl3 from theA ether. If it is desired that the AsC13 be reduced below 1% in the aqueous solution, the ether extraction processiV may be repeated vone or more times which reduces the AsCl3 in the aqueous phase to an insigniiicantly small amount.

The other half of the aqueous AsCl3 solution, which was not ether extracted Vin the above example, was reused in a ysucceeding batch as a source of HC1. This represents a substantial saving in consumption ofV fresh HCl, facilitates the slurrying of the As203, and further reduces the loss of AsCl3 remaining in the aqueous phase.

It is to be understood that other suitable organic solvents than ethers may be used in either the Semi-continuous or batch process.Y

1 The practice of the inventionoifers'a number ofadvantages, salient among which are: Vimproved economy and eiciency requiring Vlower costs and resulting in a higher quality product than that attained in heretofore known methods.

Having described ourinvention, what desire to protect by Letters Patent is:

1. A method of making high purity AsCl3 v.which ,consists essentially of (1) slurrying As203 inan aqueous HC1 solution of between V20V percent and the point of saturation, in a closed system, (2)V passing HC1 gas through the aqueous slurry so made to produce AsCl3 in suspension at a temperature of between room temperature and about 100 C., (3) allowing the reaction mixture toV cool and stratify into substantially two liquid layers: a lower. layer comprising AsCl3 and an upper aqueous layer containing dissolved AsC13, (4) separating the two layers, and (5) extracting AsCl from the thus separatedfaqueous` layer by admixing therewith an organic solvent for AsCl3, which is substantially immiscible with water, and (6) separating the solvent containing a substantial portion of the AcCl3 from the remaining aqueous portion.

2. A semi-continuous method of making high purity AsCl3 which consists essentially ofv the steps of: (1) slurrying As203 in aqueous hydrochloric acidvofbetween 20 percent and the point of saturation in a closed system, (2) passing anhydrous'HCl gas through the aqueous acid slurry at a temperature ofV between room temperature and about y100" C. Vto produce AsC13 in suspension at a temperature of between room temperature and about 100 C., and Ystratify into substantially Vtwo liquid phases: a lower phase consisting Vof at leastV about 99% AsCl3 by weight, `and an upper aqueous acidV `phase containing dissolved,

AsCl3, (4) separating thetwo phases, (5) radmixing a portionof the aqueous, acid phase with additional As203 and passing H Cl gas therethrough. to make more AfsCl3, and (6) Aextracting AsClg'from theother'portionof the aqueous acid phase with an organic solvent which is substantially .immiscible Vwith Y water Yand 'inr ywhich `A`sCl3 vis soluble, and .(7) separating the solvent containing the extracted AsC13 from the aqueous portion.Y l

3. The method according to claim 2 wherein the `solvent eontaining AsC13 extracted frornfthe aqueous AsCl3 Y weV claim and solution is subjected to distillation to remove the solvent therefrom.

4. The method according to claim 2 wherein the organic solvent is an ether.

5. The method according to claim 4 wherein the ether is diphenyl oxide.

6. The method according to claim 5 wherein the diphenyl oxide containing the AsCl3 dissolved therein is separated by volatilizing off the AsCl3.

7. A method of making high purity AsCl3 which consists essentially of (1) slurrying As203 in an aqueous HC1 solution of between 20 percent and the point of saturation in a closed system, (2) passing HC1 gas through the aqueous slurry so made at a temperature of between room temperature and about 100 C. to produce AsCl3 in suspension, (3) allowing the reaction mixture to cool and stratify into substantially two liquid layers: a lower layer of at least about 99 percent by weight of AsCl3 and an upper aqueous layer, and (4) recovering and retaining the lower AsCl3 layer and discarding the upper aqueous layer.

References Cited by the Examiner UNITED STATES PATENTS 8/45 Booth 23-98 OTHER REFERENCES 15 Millan Co., New York.

MAURTCE A. BRINDISI, Primary Examiner. 

1. A METHOD OF MAKING HIGH PURITY ASCL3 WHICH CONSISTS ESSENTIALLY OF (1) SLURRYING AS2O3 IN AN AQUEOUS HC1 SOLUTION OF BETWEEN 20 PERCENT AND THE POINT OF SATURATION, IN A CLOSED SYSTEM , (2) PASSING HCI GAS THROUGH THE AQUEOUS SLURRY SO MADE TO PRODUCE ASCI3 IN SUSPENSION AT A TEMPERATURE OF BETWEEN ROOM TEMPERATURE AND ABOUT 100*C., (3) ALLOWING THE REACTION MIXTURE TO COOL AND STRATIFY INTO SUBSTANTIALLY TWO LIQUID LAYERS: A LOWER LAYER COMPRISING ASCL3 AND AN UPPER AQUEOUS LAYER CONTAINING DISSOLVED ASCI3, (4) SEPARATING THE TWO LAYERS, AND (5) EXTRACTING ASCI3 FROM THE THUS SEPARATED AQUEOUS LAYER BY ADMIXING THEREWITH AN ORGANIC SOLVENT FOR ASCI3 WHICH IS SUBSTANTIALLY IMMISCIBLE WITH WATER, AND (6) SEPARATING THE SOLVENT CONTAINING A SUBSTANTIAL PORTION OF THE ACCI3 FROM THE REMAINING AQUEOUS PORTION. 